Frequency modulated oscillator control



Sept. 28, 1948. R. H. RANGER f 2,450,019

FREQUENCY` MODULTED4 OSCILLATOR CNTROL Filed July e, 1944 Patented Sept. 28, 1948 UNITED STATES vPATENT OFFICE (Granted under the act of March 3, 1883, as amended April 30, 1928; 370 O. G. 757) The invention described herein may be manufactured and used by or for the Government for govermnental purposes, without the payment of any royalty thereon.

In the lcase of -oscillators working at very high frequencies (VHF) a small percentage change in the frequency output amounts tol a veryy large numerical change in frequency. The means provided to hold the output frequency of the oscillator constant must be able to handle'this possibly large variation in frequency.

One object of this invention is tofprovide a control which uses frequency modulation for holding the output of VHF oscillator substantially constant at a selected frequency.

f Another object of thisV invention is to provide Imeans which operate torestore a controlled oscillator toa preselected frequency, this operation being performed with increasingV discrimination as the oscillator approaches that frequency.

Yet a further object of this invention is-to provide a control means having, as a part thereof, a frequency modulated mechanism in. which the Iintensity of lthe frequency modulation is decreased, as an oscillator under the control of said means approaches the desired frequencyv at which it is to be maintained.

Other objects of this invention will be set forth hereinafter, or-will be obvious as the descriptionproceeds.

For a further exhibition ofY my invention reference may be had to the annexed drawings and specifi-cation at the end whereof, the novel features of my invention will be specifically pointed out and claimed.

Figure 1 of the drawings is -a block diagram in which the variousparts are marked with appropriate legends.

Figure 2 is a diagram illustrating themode of operation of this device.

But one embodiment of my invention has been selected for description as numerous modifications will be readily apparent to those skilled in this art. In this embodiment, block I represents a crystal-controlled. oscillator of a convenient frequency, for example, lt lnegacycler (mc). Thereis also provided a second oscillator 2 which may conveniently have an output frequency of 1 kilocycle (kc.) A portion of the output of oscillator 2 is fed through a gain control circuit 3 toa phase shift modulation circuit 4, which is arranged so that the ou-tput of oscillator 2 shifts the phase of the frequency put out by crystalif.

Asi-swell known in. theY art, on multiplying the phase shifted output, frequency modulation, may

be accomplished. The frequency modulation circuit 4 contains an amplifying tube biased to give a very distorted output. This -output will then be very rich in. harmonics. The frequency modulated output from circuit 4 is fed to harmonic amplifier circuit 5, and a harmonic selector circuit 6 which provides means for selecting a desired lh-ar-r monie of the frequency modulated output of cr-Y cuit 4.

'Ihe oscillator whose frequency is to be oon-l trolled is shown at 1. It contains adjustment 8 shown as a capacitor although an inductor might well be used. Adjustment 8 is connected in parallel with the larger normal circuit element 9.

The outputs of oscillator 'l and harmonic selector `li .are fed to a mixer-detector circuit I0, which in conventional fashion,l provides in its ,output la beat note having a frequency which is equal to the difference between the frequencies of the outputs of oscillator 1 and harmonic selector 6, respectively. However, as willbe explained in more detail below, this beat note frequency is variable lsince the frequency of the out- .put of harmonic selectory 6 is itself variable. For convenience, this variable 'beat note will be called the primary beat. Output of mixer-detector |0 is fed to a detector or frequency-modulationdis-f criminator circuit l2. In the con-ventionalmanner, the discrimina-tor circuit converts the variations of frequency of the output of mixer-detector lo into variations. of amplitude alt a constant frequency, which may, for convenience, be calledthe secondary beat. The discriminator circuit is so designed that this secondary beat has-- the same frequency as that of oscillator 2, e. g., 1 kc.

`Let it be assumed that the harmonic frequency selected is at fo 4as shown in Figure 2. The modulation energyoscillator 2, is then connected throughV gaincontrol 3- to shiftthe phase at 4 (Figure l). and, in consequence, the selected' harmonic 6 shift-s backr and` forth from f1 to f2 as indicated above the modulationenergy trace. Asis shown here, connections are-assumed to. be Asuchthalt a positive swing of the modulation energy will cause an increase inthe frequency of the harmonic. selected. AY decrease in the frequency of the harmonic selected is caused by accurespondingA negative swing of.- the. modulationenergy.

Now, if the controlled frequency is above the selected harmonic, as indicated atya,v it is obvious that the primary beat betweenzthem will belower` Whenthe harmonicis" swung `to f2 than it will loe when the harmonic isswung to' f1. Inother' words, the ensuing primary beat will be inversely proportional in frequency to the modulation energy if the frequency to be controlled is above the harmonic selected as it is as shown at fs.

C-ontrarywise, if the frequency to be controlled is below the selected harmonic as shown at f4 the primary beat will be proportional in frequency t-o the modulation energy swing, as excursions of the selected harmonic to the right at f2 will increase the frequency of the beat between it and 'the oscillator at f4.

It is n-ow necessary to have a discriminator which will indicate these frequency changes in the primary beat. This is accomplished in the discriminatcr I2 (Figure 1). As is well known in the art, it is easy to arrange such a discriminator circuit. For example, a capacitive coupling in any of `the amplifier sta-ges would give an increased ouftput for increased frequency, whereas an inductively reactance coupling would give a decreasing energy response f-or increasing frequencies.

Let it Ibe assumed that a capacitive coupling is used which will give an increasing response for increasing beat frequency.

The output of the discriminator I2 will, as stated above, be Ithe same as that caused by the modulation energy yof oscillator 2. The beat note pattern of the output of mixer-detector IIl will be the same, for a certain variation of the frequency of controlled oscillator l in an upward direction, as `the beat note pattern for the same variation ina downward direction. The two beat note patterns will, however be 180 out of phase with each other. One of these beat note patterns would 'be in phase with the output of oscillator 2 while the other would be 180 out of phase with this output. As has already been stated, discriminator I2 converts the vari-able beat note frequency output of mixer-detector I into a constarrt frequency. In addition, discriminator I2 operates in conventional fashion to recognize the phase of its input. Accordingly, with one beat note pattern, it produces a first output with one phase relation which may be assumed to be in phase with the output of oscillator 2, while with the other beat note pattern the output of discriminator I2 is 180 out of phase with the first output and therefore 180 out of phase with the output of oscillator 2.

By means of the 90 degree phase shift I5, the output of the oscillator 2 is assumed shifted forward 90 degrees to act as a reference frequency in one set of coils of a two-phase motor I3. This two-phase motor I3 will then rotate in one direction or the other when the output of I2 is also applied to its other set of coils, depending upon the existing phase relationship of the output of I2 and the oscillator 2.

The net result is that the two-phase motor I3 will rotate in one direction or the other as the phase of the secondary beat reverses, depending upon whether the controlled frequency put out by oscillator 'I is above or below the selected harmonic. Thus, it is only necessary to have motor I3 drive adjustment 3 (in this case a trimmer capacitor) in such a direction that an in-phase secondary beat will move the motor in a direction to decrease the trimmer capacitance, whereas an out-of-phase secondary beat will cause motion in the opposite direction and increase the trimmer capacitance across that of the main tun- `.ing capacitor 9 of the controlled oscillator 1.

When the frequency of the controlled oscillator 'I centers at the average of the frequency 1.1191131 lated position of the selected harmonic then the secondary beat will be at double the modulation frequency, and therefore there will be practically no secondary beat at the modulated frequency, and the correction motor will come to rest.

In order to provide a control for a wide band of frequencies and at the same time to provide motor I3 with suicient power as the controlled oscillator 1 approaches the frequency for which it is set, an automatic volume control circuit I6 is provided. This circuit works in two ways, both of which depend upon the decrease-in frequency of the primary beat.

I'Due to the capacity coupling II between the primary beat output from mixer-detector I0 to the automatic volume control I6, the energy transfer will decrease as the primary beat decreases in frequency. This decrease is reflected in automatic volume control I6 to decrease the phase shift gain control 3, so that the final frequency shift in the output of harmonic selector 6 will decrease as the primary beat decreases, which comes about as the oscillator 2 is approaching the right frequency. But by the same token, the power of the secondary beat output from the discriminator I2 would likewise decrease. To compensate for this, the decreasing energy entering automatic volume control I6 is made to increase the gain inversely in the discriminator I2. The purpose of this coupling to the automatic volume control at I6 is to see to it that as the primary frequency beat decreases in intensity that there is a corresponding automatic volume control in IS which will decrease the gain in 3 and conversely increase the gain in I2. It is, of course, obviously possible to reverse the effective control on `I2 and 3 of the automatic volume control I6, one down and the other up, as is well known in the art.

The arrows in all the circuit connections of the drawingsshow the way in which the action progresses.

I claim:

1. A process of controlling the frequency of a high frequency oscillator over a wide range of frequencies, said process comprising, generating a first electrical Wave of substantially constant frequency, generating a second electrical wave of substantially constant frequency, shifting the phase of said first wave by modulating the frequency of said first wave by means of said second wave, amplifying and selecting a harmonic of said phase-shifted wave, generating a third electrical wave whose frequency is to be controlled, mixing said amplified and selected harmonic with said third wave and thus detecting a primary beat note of variable frequency, converting the primary beat note into a secondary beat note of the same frequency as the second wave, and combining energy from the secondary beat note and from the second wave in phase quadrature to each other to provide control energy to cause the frequency of the third wave to be brought to its proper frequency in the event of any variation from such proper frequency.

2. A process of controlling the frequency of a high frequency oscillator over a wide range of frequencies, said process comprising, generating a first electrical wave of substantially constant frequency, generating a second electrical wave of substantially constant frequency, shifting the phase of the said first Wave by modulating the frequency of said first wave by means of ysaid second wave, generating a third electrical wave whose frequency is to be controlled, mixing said third wave Said frequency-shifted wave, thus secondary beat note of the same frequency as the 'second Wave, and combining energy from the secondary beat note and from the second Wave in phase quadrature to each other to provide control energy to cause the frequency of the third Wave to be brought to its proper frequency in the event of any variation from such p-roper frequency and causing said second Wave to-vary the modulation intensity of said frequency modulation in direct ratio to the difference in frequency between said third Wave and said frequencyshifted wave.

3. A process of controlling the frequency of a high frequency oscillator over a Wide range of frequencies, said process comprising, generating a rst electrical wave of substantially constant frequency, generating a second electrical Wave of substantially constant frequency, shifting the phase of the said first wave by modulating the frequency of said rst Wave by means of said second Wave, generating a third electrical wave Whose frequency is to be controlled, mixing said third Wave and said frequency-shifted Wave, thus detecting a primary beat note of variable frequency, converting the primary beat note into al an oscillator, comprising: generating a rst elec- L trical Wave of substantially constant frequency, generating a second electrical Wave of substantially constant frequency, frequency-modulating the rst Wave by means of the second Wave, generating a third electrical Wave whose frequency is to be controlled at a certain normal value, mixing the third wave and the frequency-modulated Wave and thus detecting a primary beat note of variable frequency, converting the primary beat note into a secondary beat note of the same frequency as the second wave, and combining energy from the secondary beat note and from the second Wave in phase quadrature to each other toprovide control energy to bring the frequency of the third Wave to its normal value spective inputs; discriminator means whose inf put is derived from the output of the mixerdetect-or means and 'whose output has a constant frequency; and frequency-adjusting means deriving input from the discriminator means and from the frequency-modulating means and thus being ,responsive tol afvariation of the controlled frequency from its nor-mal value, such frequencyadjusting means being connected to the tuning means andoperating the tuning means to bring the controlled frequency to its normal value, in the event of its variation from such normal value.

6. Means for controlling the frequency of an oscillator comprising: reference oscillator means, such means including a reference oscillator and including also frequency-modulating means which convert output of the reference oscillator into frequency-modulated energy With a predetermined mean frequency value, such frequency-modulating means comprising modulating oscillator means having a predetermined constant frequency; controlled oscillator means Whose frequency normally has a predetermined constant value and which include tuning means; mixing-detector means supplied with energy from the reference oscillator means and the controlled oscillator means, and producing an output with e beat frequency equal to the difference between the frequencies of the energy from the reference oscillator means and the controlled oscillator means, respectively; discriminator means supplied Iwith energy from the mixer-detector means and having a constant frequency output; and motor means which derive input from the modulating Yoscillator means and the discriminator means, respectively, and which combine the respective inputs in phase quadrature to each other, such motor means being connected to and serving to operate the tuning means to bring the controlled frequency to its normal uvalue, in the event of its variation from such normal Value.

7. Means for controlling the frequency of an oscillator, as described in claim 6, in which the frequency-modulating means include phaseshifting means deriving input from the reference oscillator and the modulating oscillator means, and include also means for amplifying and selecting a harmonic of the output of these phaseshifting means.

8. Means for controlling the frequency -of an oscillator, as described in claim 6, which include automatic volume control means deriving input from the mixer-detector means and supplying energy to the modulating oscillator means to cause the frequency deviation of the output of the reference oscillator means to become progressively smaller as the extent of the variation of the controlled frequency from its normal yalue becomes smaller.

9. Means for controlling the frequency of an oscillator, as described in claim 6, which include automatic volume control means deriving input from the mixer-detector means and supplying energy to the discriminator means in such manv ner as to com-pensate for the tendency of the output of the discriminator means to decrease With decrease in the output of the mixer-detector means, as the extent of the variation of the controlled frequency from its'normal value becomes smaller.

10. Means for controlling the frequency of an oscillator, as described in claim 6, which include automatic volume control means deriving input from the mixer-detector means and supplying energy to the modulating oscillator means to cause the frequency deviation of the output of the reference oscillator means to become progressively smaller as the vextent of the variation of the controlled frequency from its normal value becomes smaller; such automatic volume control means also concurrently supplying energy to the REFERENCES CITED The following references are of record in the fue of this patent:

Number UNITED STATES PATENTS Name Date Ohl May 26, 1936 Logan Mar. 21, 1939 Goldstine Sept. 19, 1944 

